There is provided a zoom lens system, compact and light in weight and having high image formation performance over the zoom entire range irrespective of object lens while covering the range from the broad angle side up to the telescopic end state side, which is used in video camera and/or digital still camera. A zoom lens system at least includes a first lens group GR1 having positive refractive power, a second lens group GR2 having negative refractive power, a third lens group GR3 having positive refractive power, a fourth lens group GR4 having positive refractive power, a fifth lens group GR5 having weak refractive power (including refractive power=0), and a sixth lens group GR6 having negative refractive power which are arranged in order from the object side to change spacings between the respective lens groups to thereby perform magnification changing or adjusting operation, wherein when group spacing between the i-th lens group and the j-th lens group at the wide-angle end state is DW(i-j), and group spacing between the i-th lens group and the j-th lens group at the telescopic end state is DT(i-j), the following conditional formulas (1), (2) and (3) are satisfied, and the fourth lens group GR4 is moved in the optical axis direction so that focusing operation is performed.
DW(1−2)<DT(1−2) (1)
DW(2−3)>DT(2−3) (2)
DW(5−6)>DT(5−6) (3)